Early nicotine exposure during brain development produces long-lasting behavioral changes that are detrimental in multiple ways. These include greater propensities for nicotine addiction, attention deficit hyperactivity disorder, anxiety, and depression. The mechanisms remain unclear. We have preliminary evidence indicating that early exposure of mouse pups to nicotine from the lactating mother during nursing (postnatal day 2-16) produces long-lasting increases in the number of glutamatergic synapses and increases in the ratio of excitatory-to-inhibitory synaptic input neurons receive. The preliminary results also indicate that even after a long period of nicotine abstention, the mice as adults have abnormally large numbers of neurons that display high levels of activity when re-challenged briefly with nicotine. This can be seen either in acute slices or in alive animals with in vivo imaging. Such changes at the cellular level are new and likely to contribute importantly to the long-lasting behavioral changes reported. To examine the underlying mechanisms and evaluate their consequences, we propose the following. We will use immunostaining and patch-clamp recording from neurons in acute slices to determine the extent of increases in the glutamatergic input they receive after nicotinic exposure from the mother via nursing or in utero. We will determine whether the changes extend into adulthood long after nicotine cessation, and whether such animals are more vulnerable to network changes when challenged with subsequent nicotine than are nave animals. Further, we will determine whether subpopulations of neurons under these conditions can be identified by expression of the immediate early gene c-fos, characteristic of high activity, and we will also use in vivo imaging of awake mice to examine the properties of hyper-active cells. Neuronal populations to be examined include pyramidal neurons in the hippocampal CA1 because of their roles in memory formation and dopaminergic neurons in the ventral tegmental area because of their participation in reward pathways. These studies will identify mechanisms and pathways contributing to the long-lasting effects of early nicotine exposure. They will provide new insight into mechanisms guiding important aspects of circuit formation and brain development. Importantly, they will also have significant biomedical relevance because of current medical policy recommending nicotine replacement therapy for pregnant women who smoke. That procedure, together with the increasing usage of electronic cigarettes, pose serious health threats that are insufficiently understood. The results obtained here will help clarify the consequences and indicate new strategies for exploration.